This is why whitespace should not be syntax. One is a syntax error, one is not. Can you guess?

foo.go

    for i, j := 0, len(r)-1;
        i < len(r)/2;
        i, j = i+1, j-1
    {
        r[i], r[j] = r[j], r[i]
    }
    
    for i, j := 0, len(r)-1;
        i < len(r)/2;
        i, j = i+1, j-1 {
        r[i], r[j] = r[j], r[i]
    }